GeForce GT 640..
 

EVGA GeForce GT 640 901MHZ 2GB 1782MHZ DDR3 2xDVI Mini-HDMI HDCP DirectX 11 Video Card or
EVGA GeForce GT 640 901MHZ 2GB 1782MHZ DDR3 2xDVI Mini-HDMI HDCP PCI-E DirectX 11 Video Card
These seems to be a bargain but I'm not sure if my PC can handle this.
My PC specs:
500 Cooler Master RS PCAR-A3
AMD phenom2 x4 965
4 gigs of ram

The phenom II cpus are still potent chips. Probably comparably to Vischera for single threaded performance (gaming). You'll have no problem running that video card.

A GTX 640 is far from being a super power card, it is a fair bit weaker than the card I got with my Phenom II 940 back when it was fairly new... So, your Phenom II 965 will have more than enough power for such a card. Now, is that a good price or are you better off buying a card used... I would probably opt for the 2nd option, but that depends on what u wanna do.

What are you planning to do with the card? If you're not gaming or doing anything else with 3D or GPGPU, I say get an even cheaper DX11 card, like the Radeon 5450s that go on sale for $30 every so often.
If you are gaming, you're better off with SLIGHTLY pricier cards:
Powercolor Radeon HD 7750 800MHZ 1GB 4.5GBPS GDDR5 DVI HDMI DisplayPort PCI-E Video Card - DirectCanada
Newegg.ca - SAPPHIRE Radeon HD 7750 1GB 128-bit GDDR5 PCI Express 3.0 x16 HDCP Ready Video Card ( 11202-00-20G)

When you're into the price brackets that you're skirting with those cards, $15 to $20 makes a world of difference.

I only have a 500watts power supply.